There is increasing evidence that 4-hydroxyestradiol mediates the induction of cancer by estradiol (E2) in animals and possibly breast and other hormone-associated cancer in humans. This pathophysiological role of 4-hydroxyestradiol is supported by its specific biosynthesis in tissues of rodents where E2 induces tumors or in human breast and uterine tissue and by the induction of DNA damage and of tumors in rodent tissues by 4-hydroxyestradiol. Known physiological roles of 4-hydroxyestradiol include induction of blastocyst implantation in the uterus and upregulation of uterine lactoferrin gene expression 60 fold over vehicle controls compared to less than a doubling by E2. These data indicate a hormonal role of 4- hydroxyestradiol distinct and different from that of E2. Our hypothesis is that 4-hydroxyestradiol acts as a hormone binding to its own receptor. This 4-hydroxyestradiol receptor-mediated hormone action may regulate cell proliferation and other biological events in the uterus and mammary gland. Loss of cellular control over 4-hydroxyestradiol formation and catabolism may induce pathophysiological consequences resulting in tumor initiation and growth in the uterus, breast and elsewhere. In order to examine this hypothesis, we first need to isolate the putative 4-hydroxyestradiol receptor and characterize it. Specifically, we plan to: 1. Synthesize 4-hydroxyestradiol-linked sepharose 6B and use it to isolate the receptor from mouse uterine tissue, and 2. isolate sufficiently pure protein for a partial sequence, which will be used to obtain a partial gene sequence. This plan is in line with goals 1 and 3 of the Breast Cancer Progress Review Group to better understand the biology of the normal mammary gland and of precancerous lesions.